The present invention relates to a developing device for developing an electrostatic or magnetic latent image with a toner developer agent, and to a multi-color recording apparatus using this developing device.
A multi-color recording apparatus of the kind referred to above is employed in, for example, an electrophotographic system. Generally, the recording apparatus adopts a system where a process, in which a latent image is formed on a photoconductive material by charging and exposure and subsequently a toner of colored powder is used to develop the latent image, is repeated plural times while the photoconductive material is rotated plural times.
In order to raise the speed of the multi-color recording, there is known an apparatus in which a single photoconductor is used as a latent-image carrier, and a plurality of developing devices corresponding respectively to the colors are arranged about the photoconductor to form a multi-color image during one revolution of the photoconductor. This apparatus is disclosed in, for example, in Japanese Patent Unexamined Publication No. 52-106743 and Japanese Publication No. 43-19197.
In the apparatuses disclosed in the above publications, the developer agent including a colored toner is brought into contact with the surface of the rotating photoconductor in each developing device. As a result, during the second and subsequent developments, the toner applied to the surface of the photoconductor at the first development is brought into contact with the developer agents at the second and subsequent developing devices, and is scraped off. This raises problems of a decrease in concentration or density of the print image and a mixing of the toner having a different color into the second and subsequent developing devices.
On the other hand, in order to prevent the above-mentioned scratching or scraping of the image, a method is known, in which the toner developer agent is maintained out of contact with the surface of the photoconductor, and the toner particles are jumped to the latent image during the developing process, as disclosed in, for example, U.S. Pat. No. 3,702,483. In addition, U.S. Pat. application Ser. No. 159,259 filed on Feb. 23, 1988 in the name of the inventors of this application and others discusses a reduction or a variation in the concentration or density of the image occurring in the non-contact development, and contamination of recording sheets due to adhesion of unnecessary toner particles. The inventors of this application and the others have proposed a developing device in which a magnetic-pole pitch of the developer roll is set in a specific condition to increase an area to be flown in toner, or bias voltage for forming an electric field for jumping of the toner is adjusted in accordance with a change in the developing gap, to increase and stabilize the concentration or density of the image.